MEGA: Spectrophotometric SED Fitting of Little Red Dots Detected in JWST MIRI

2025/08/27 by Kaila Ronayne, Casey Papovich, Ronayne, Kaila +29 · 1 citation
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena

paper · pdf · doi:10.48550/arxiv.2508.20177

openalex publication_date 2025/08/27 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28

Abstract

We analyze eight spectroscopically confirmed Little Red Dots (LRDs) at redshifts z = 5.1-8.7 with JWST/NIRCam, NIRSpec, and MIRI data. The LRDs have red NIRCam colors, F150W-F444W > 1, but flat NIRCam-MIRI colors, -0.5 < F444W - F770W < 0.5, suggesting weak warm/hot dust components. The LRDs have -1.0 < F1000W - F1500W < 1.1, suggestive of non-uniform rest near-IR properties within the sample. We model the spectral energy distributions (SEDs) of the LRDs using the CIGALE and Prospector codes to assess how the differing templates impact the interpretation for LRDs for cases of: (1) models with star-forming stellar populations only; (2) active galactic nuclei (AGN) dominated models; and (3) composite AGN and star-forming models. Using the Bayesian information criterion, we find that six of the eight LRDs favor AGN models compared to star-forming models, though no model reproduces all of the observed properties. Two LRDs with pronounced Balmer-breaks and broad Hα have SEDs that are reproduced with hot, dense-gas (log T/K=5-5.7, log n/\mathrmcm^-3 = 9-11) models with low dust attenuation (A(V)≃ 0.5 mag). However, these models require an additional thermal component (800-1400 K) to account for the MIRI data, and fail to reproduce the rest-UV and narrow [OIII] emission. The total bolometric emission from the dense-gas models, and possibly CIGALE AGN models, appear consistent with literature constraints in the far-IR and radio, and require log L_bol/L_\odot<12. These results suggest that our LRDs cannot be modeled entirely with standard templates, but instead require a novel treatment of gas conditions, AGN and star-formation.

Citations

• The Discovery of Little Red Dots in the Local Universe: Signatures of Cool Gas Envelopes
• Dust Budget Crisis in Little Red Dots
• CAPERS-LRD-z9: A Gas Enshrouded Little Red Dot Hosting a Broad-line AGN at z=9.288
• CEERS: Forging the First Dust Grains in the Universe? A Population of Galaxies with spectroscopically-derived Extremely Low Dust Attenuation (GELDA) at 4.0
• A remarkable Ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a Little Red Dot at z=3.5
• A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots

Cited by

• Discovery of Multiply Ionized Iron Emission Powered by an Active Galactic Nucleus in a z~7 Little Red Dot

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